Patent application WO 2004/040899 discloses techniques to modify, in an imperceptible manner for the human eye, the spectrum of the images displayed by a video projector in order to deter the copy of images using a camcorder. The modification of the spectrum is obtained by using a fourth primary colour component. This fourth primary is used to create spectra having different spectral compositions but inducing the same light and colorimetric perception by the human eye. Such spectra exist for each colour since the human eye only uses three types of receivers (or cones) to create the colorimetric sensation associated with a light. Spectra producing identical effects on the three types of receivers appear identical to the eye without necessarily being identical. Such spectra are called metameric. We also speak of metameric colours or metameric mixtures. Camcorders generally having RGB light receivers noticeably different from those of the human eye, metameric spectra for the human eye are not necessarily metameric for the camcorder and appear as different with respect to chrominance or luminance on the copy filmed by the camcorder. This effect is then used to insert watermarking or anti-copy marks in the projected images to degrade the quality of the copies made by the camcorder and possibly know the origin of the copy. The watermarking is for example a message indicating that the film is an illicit copy or a code identifying the film which has been copied or the cinema that has projected the film. This patent application also gives a method to calculate the proportions of each of the primary components to make it possible to obtain different mixtures having the same colorimetric coordinates in a visual colour space as a given light that can be obtained by three primaries.
Patent application US 2006/0152524 discloses also metameric techniques that are adapted to deter the copy of images using a camcorder. According to paragraph 35 of this document, pixels or groups of pixels (i.e. image portions) that can be watermarked are randomly selected or selected among the image portions having high spatial detail, without consideration about the efficiency of the metameric jamming itself for a camcorder.
Problems inherent in the use of metameric spectra nonetheless exist. All the spectators do not necessarily have the same colour perception. Hence, a watermarking which should normally be invisible for all the spectators can be visible for some of them. Moreover, if the fourth primary is produced by an additional projector, there may be a geometrical alignment problem for the four primaries. Therefore, the impact of these problems should be reduced so that the watermarking insertion does not disturb the spectators present in the cinema. Moreover, it is advisable to enhance the efficiency of the metameric jamming itself for a camcorder.